Pre-cleaning of silicon wafers in semiconductor manufacturing is carried out in order to remove a native oxide from the surface of a crystalline silicon wafer prior to deposition or ion implantation steps. Removal of the native oxide is best carried out using plasma processing because it tends to have higher throughput and better uniformity than other techniques. The plasma process version of the wafer pre-clean step can take the form of a reactive ion etch process, with etch precursor gases (e.g., gases containing fluorine or other etch species) being introduced into the chamber. The process requires high throughput because the native silicon dioxide layer to be removed is on the order of 100 Angstroms thick and covers the entire wafer diameter (which may be 200 mm, or 300 mm or 400 mm). The process requires a high degree of uniformity because the native oxide film must be thoroughly removed from the entire wafer without etching the underlying crystalline silicon wafer surface. Therefore, there can be no appreciable deviation in the radial distribution of etch rate across the wafer surface. We have found that an inductive source (an overhead external coil) driven at a low RF frequency (e.g., about 2 MHz) in combination with an HF bias (e.g., about 13.56 MHz) coupled to the wafer through the support pedestal provides the desired etch performance of both high throughput (etch rate) and high uniformity of etch rate radial distribution across the wafer surface.
Attainment of this goal has been hampered by extreme variations in etch rate uniformity among plasma pre-clean reactors of apparently identical design. In producing and testing plasma pre-clean reactors, we have found that while one reactor may provide outstanding uniform radial etch rate distribution across the wafer surface, another reactor of the same design produces such highly non-uniform etch rate distribution that the reactor is not useful. We have found that this problem can be ameliorated on an ad hoc basis by changing the length of the RF coaxial cable driving the inductive coil antenna of a poorly performing reactor. For example, a reactor with poor etch rate uniformity whose nominal design called for a particular length 50-Ohm coaxial cable connected between the source power RF generator output and the coil antenna (e.g., 100 foot, 75 foot or 50 foot length) could be dramatically improved by substituting a different length coaxial cable. For example, one reactor was improved by a change in cable length corresponding to a phase shift of about ⅙ wavelength at the bias power frequency. We deduced from this that the inferior etch uniformity of the poorly performing pre-clean reactors was attributable to the presence of frequencies in the inductive coil antenna other than the source power frequency, i.e., the bias power frequency and frequencies obtained by mixing the source power with the bias power. We confirmed this experimentally by observing the presence of sums and differences of the bias and source power frequencies at the source power RF generator output (coupled back from the plasma through the coil antenna and through the coaxial cable). It is felt that the plasma inside the reactor chamber acts as a mixer of the source and bias frequencies to produce intermodulation products including the fundamental and harmonics of the bias frequency and their sum and difference frequencies (“sidebands”) with the source power frequency. These intermodulation products are coupled through the plasma back to the inductive coil antenna.
At first, it was noticed that the presence of frequency components containing the HF bias frequency coupled from the plasma to the coil antenna caused an erroneous indication in the reflected power sensing circuits of the RF generator, causing the generator's power level to be servoed away from a desired level. It is desirable that the reflected power at the generator output constitute less than 5% of the output power. Poor etch performance, due to loss of control over RF power and uncertainty as to the actual applied RF voltage, occurs when the reflected power ratio exceeds 5% (e.g., 10% or more). The plasma-coupled HF frequency components are sensed as reflected power by the LF source power generator's SWR bridge. The magnitude of this problem varied depending upon the manufacturing source of the RF generator, making reactor performance dependent upon the brand of RF generator. This problem was solved by installing a low pass filter at the generator output to block the HF bias frequency and its sidebands with the source power frequency, thereby preventing these components from affecting the generator's SWR bridge. While this solution removed the problem of loss of control over RF power level, it represents a costly modification of an RF generator and it does not fully solve the problem of poor etch rate distribution uniformity.